A critical review on whether reducing the bandgap width is the sole approach to enhancing photocatalytic efficiency in g-C3N4-based materials.

Graphitic carbon nitride (g-C3N4) is a metal-free photocatalytic material that has garnered significant research interest. Bandgap narrowing has long been regarded as a key strategy for enhancing photocatalytic efficiency. This review presents a systematic overview of strategies for bandgap tuning in g-C3N4, including elemental doping, defect engineering, and the construction of heterostructures. It also elucidates the mechanisms by which bandgap narrowing improves performance. Furthermore, this review demonstrates that bandgap widening caused by quantum confinement effects can enhance photocatalytic activity by facilitating charge separation and strengthening redox driving forces. This finding dispels the long-standing misconception that "narrower bandgaps lead to higher efficiency." This review further reveals that excessive doping or defects can introduce recombination centers and cause structural instability, resulting in a nonlinear relationship between the bandgap and efficiency. It also emphasizes the synergistic role of energy level alignment, carrier dynamics, and reaction specificity and proposes a comprehensive optimization framework that extends beyond the regulation of a single bandgap. These findings provide valuable theoretical insights into the rational design and practical application of high-performance g-C3N4-based photocatalysts in the fields of environmental remediation and clean energy conversion.